1. Field of the Invention
The present invention relates to an electroconductive paste, a method of producing a monolithic ceramic electronic part in which the conductive paste is used to form inner conductor films, and a monolithic ceramic electronic part made from the conductive paste and in particular, to an improvement in that structural defects can be suppressed even if layers contained in a monolithic ceramic electronic part are thinned and the number of the layers is increased.
2. Description of the Related Art
Monolithic ceramic capacitors as examples of monolithic ceramic electronic parts are generally produced by the following method.
First, ceramic green sheets containing a dielectric ceramic raw material and having inner conductor films formed in required patterns on a surface of the sheets by use of a conductive paste containing a conductive component are prepared. For example, a material containing BaTiO3 as a major component is employed as the dielectric ceramic raw material.
A plurality of ceramic green sheets including the above-described ceramic green sheets having the inner conductor films formed thereon are laminated and hot-press bonded. Thus, an integrated green laminate is produced.
Subsequently, the green laminate is fired. Thereby, a sintered laminate is obtained. The laminate has a lamination structure containing a plurality of the ceramic layers which are made form the above-described ceramic green sheets. The above-described inner conductor films are arranged via the ceramic layers inside the laminate so that an electrostatic capacitance can be generated.
Then, outer electrodes are formed on the surface of the laminate to be electrically connected to specific ones of the inner conductor films for use of the static capacitance.
Thus, a monolithic ceramic capacitor is produced.
In recent years, the ceramic layers of such monolithic ceramic capacitors have been further thinned and the number of the layers has been increased for the purpose of reducing the size and increasing the capacitance.
To thin the ceramic layers and increase the number of the ceramic layers, it is important to sufficiently harmonize the shrinkage behaviors during firing of the ceramic layers and the inner conductor films with each other.
Ordinarily, the shrink-starting temperatures of conductive metallic powders contained in inner conductor films are considerably lower than those of the ceramic layers. In the case in which there are differences between the shrink behaviors of the conductive metallic powders and the ceramic layers, relatively large stresses are generated inside the monolithic ceramic capacitors, so that the thermal impact resistances are reduced, and seriously, cracks and peeling may be caused between the ceramic layers and the inner conductor films.
To solve the above-described problems, for example, Japanese Unexamined Patent Application Publication No. 6-290985 has proposed a method of causing the shrink behavior of the inner conductor films to approach that of the ceramic layers as much as possible. According to that method, different types of ceramic raw material powders of which the compositions are the same as or different from those of ceramic raw materials contained in the ceramic layers are added to conductive pastes for forming the inner conductor films.
According to the above-described Japanese Unexamined Patent Application Publication No. 6-290985, oxides of Zr, rare earth elements and the like are added to a conductive paste to suppress the conductive metallic powder contained in the conductive paste from sintering, so that the shrink behavior of the inner conductor films can approach that of the ceramic layers. Thereby, successfully, cracking and peeling between the ceramic layers and the inner conductor films are suppressed.
In addition to Japanese Unexamined Patent Application Publication No. 6-290985, for example, Japanese Examined Patent Application Publication No. 5-63929, Japanese Unexamined Patent Application Publication Nos. 2001-15375, 2000-269073, and 6-969998 and so forth, describe that ceramic raw material powders are added to conductive pastes for forming inner conductor films. In these Patent Specifications, it is described as advantages of the inventions that the dielectric constants are increased, and the coverage of inner conductor films is enhanced in addition to the prevention of structural defects in monolithic ceramic capacitors.
With the recent advancement of electronics, the size of small electronic parts has been remarkably reduced. Also, for monolithic ceramic capacitors, it has been required to further reduce the size and increase the capacitance. For example, monolithic ceramic capacitors having a ceramic layer thickness of about 2 xcexcm are about to be provided for practical applications.
Regarding the inner conductor films, film-thicknesses of about 1 to 2 xcexcm are employed in most cases. Accordingly, the thickness of each of the ceramic layers becomes nearly equal to that of each of the inner conductor films. As a result, the problems caused by the difference between the shrink behaviors at firing of the ceramic layers and the inner conductor films become more serious. Thus, structural defects are readily caused in the monolithic ceramic capacitors.
From the standpoint of the principle of material diffusion, it is supposed that some reaction occurs between ceramic raw materials contained in a conductive paste for forming inner conductor films and components present on the ceramic layer side. For example, according to the method described in Japanese Unexamined Patent Application Publication No. 6-290985, structural defects in the monolithic ceramic capacitor can be suppressed. However, a metal oxide, which is not a major component of the ceramic layers, is added to the conductive paste, and therefore, the metal oxide contained in the conductive paste and a component contained in the ceramic layers react with each other. This may change the electrical characteristic of the ceramic layers.
If the above-described reaction occurs uniformly, the problems will not become serious. However, since the reaction occurs unevenly as a practical matter, the electrical characteristics of the ceramic layers are locally changed. This causes the electrical characteristics of the monolithic ceramic capacitor to disperse.
In particular, as described in Japanese Patent No. 2722457, when an oxide of a rare earth element is added to a conductive paste, the part of the ceramic layers in contact with the rare earth element oxide powder becomes semi-conductive. Thus, the thickness of the part of the ceramic layers which practically function as a dielectric is smaller than the apparent thickness of the ceramic layers. As a result, the reliability of the insulation resistance and the other electrical characteristics of the monolithic ceramic capacitor is deteriorated. Therefore, the method in which the rare earth element oxide is added cannot correspond to the thinning of the ceramic layers.
In the case in which a conductive paste having rare earth element oxides added thereto is used, advantages such as increase of the dielectric constant, enhancement of the reliability and so forth, can be obtained by devising the application method, as described in Japanese Examined Patent Application Publication No. 5-63929 and Japanese Unexamined Patent Application Publication No. 2001-15375. However, a major component of the ceramic layers and this major component of the conductive paste, which are metal oxides different from each other, react with each other at random. Therefore, the electrical characteristics of the ceramic layers are dispersed. This causes the electrical characteristics of the monolithic ceramic capacitor as a product to be disperse.
To cope with the above-described dispersion, products may be selected for shipment so as to comply with the standards for the respective characteristics. In this case, the yield in mass-production is low. The defective proportion is high. The high cost is also a problem.
It is estimated that a thickness of up to 1 xcexcm will be employed in future. Thus, if further thinning of the ceramic layers is realized, the effects of the dispersion will be more remarkable. Thus, the above-described problems will become more serious.
In case in which a metal oxide such as a rare earth element oxide, which is different from a major component of ceramic layers, is added to a conductive paste for forming inner conductor films, it may be more effective if the metal oxide is added not singly but together with the major component of the ceramic layers or a component analogues to the major component.
However, when the methods described in Japanese Unexamined Patent Application Publication No. 2001-1537 and Japanese Unexamined Patent Application Publication No. 2000-269073 are used while the above-described technique is applied, the rare earth element oxide diffuses into the ceramic layers and reacts with components of the ceramic layers. This causes the electrical characteristics of the monolithic ceramic capacitor to disperse, resulting in deterioration of the yield in mass production and an increase of the proportion of defectives.
Under the above-described situations, it is desired to develop a conductive paste for forming inner conductor films which cause no structural defects in monolithic ceramic capacitors and exerts substantially no undesired influences over the electrical characteristics of the ceramic layers, even if the sizes of the monolithic ceramic capacitors are reduced more, and the capacitances are further increased.
The above-description is true of other monolithic ceramic electronic parts in addition to the monolithic ceramic capacitor.
Accordingly, it is an object of the present invention to provide an electroconductive paste, a method of producing a monolithic ceramic electronic part in which the conductive paste is used to form inner conductor films, and a monolithic ceramic electronic part formed by use of the conductive paste.
This invention has been devised by the inventors of the invention, based on the following knowledge.
That is, to cause the shrink behavior of the inner conductor films at firing to approach that of the ceramic layers, it is effective that the grains of a ceramic powder finer than those of a conductive metallic powder contained in a conductive paste for forming the inner conductor films, and are uniformly distributed between the grains of the conductive metallic powder contained in the dried conductive paste before firing. Thereby, effects by the addition of the ceramic powder can be realized. Thus, the required amount of the ceramic powder to be added to the conductive paste can be suppressed and minimized.
When a metal oxide, which is not a major component of the ceramic layers, is added to the conductive paste for forming the inner conductor films, the metal oxide is added not singly but together with the major component of the ceramic layers or a component analogous to the major component. However, the components of the ceramic layers are not directly mixed and added to the conductive paste. Preferably, they are previously heat-treated, so that they become impossible to be sintered at the sintering temperature of a ceramic used as the substrate, and thereafter, are added to the conductive paste.
The conductive paste of the present invention devised based on the above-described knowledge is used to form inner conductor films of a monolithic ceramic electronic part, which contains a plurality of ceramic layers composed of substrate ceramic layers and the inner conductor films extending on specific boundaries between the ceramic layers. Characteristically, the conductive paste has the following constitution.
That is, the conductive paste contains a conductive metallic powder, a ceramic powder and an organic vehicle.
The ceramic powder is a powder produced by calcining an ABO3 system ceramic in which A represents Ba or alternatively Ba partially substituted by at least one of Ca and Sr, and B represents Ti or alternatively Ti partially substituted by at least one of Zr and Hf, the system containing at least one selected from the group of consisting of Re compounds in which Re represents at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu and Y, Mg compounds and Mn compounds. The ceramic powder has an average grain size smaller than that of the metal powder and is incapable of sintering at the sintering temperature of the substrate-use ceramic.
Preferably, the metallic powder is at least one selected from the group of Ag, Ag-base alloys, Ni, Ni-base alloys, Cu, and Cu-base alloys.
According to the present invention, there is provided a method of producing a monolithic ceramic electronic part having a plurality of ceramic layers made of a substrate ceramic and inner conductor films extending specific boundaries between the ceramic layers.
The method of producing a monolithic ceramic electronic part comprises the steps of preparing a ceramic green sheet containing as a major component a substrate ceramic raw material powder having the general formula Axe2x80x2Bxe2x80x2O3 in which Axe2x80x2 represents Ba or alternatively Ba partially substituted by at least one of Ca and Sr, and Bxe2x80x2 represents Ti or alternatively Ti partially substituted by at least one of Zr and Hf, and laminating a plurality of the ceramic green sheets so as to form the ceramic layers to produced a green laminate in which the above-described electroconductive paste is provided on specific boundaries between the ceramic green sheets to form the inner conductor films, and firing the green laminate.
Preferably, this production method is applied to a method of producing a monolithic ceramic capacitor. In this case, the method further comprises the step of arranging the inner conductor films to be formed by use of the conductive paste via the ceramic layers in such a manner that a static capacitance can be generated, and after the firing step for the green laminate, forming outer electrodes on the outer surface of the sintered laminate in such a manner that the outer electrodes are electrically connected to specific ones of the inner conductor films to use the static capacitance.
Further, according to the present invention, there is provided a monolithic ceramic electronic part which comprises a plurality of ceramic layers made of a substrate ceramic and inner conductor films extending specific boundaries between the ceramic layers.
In this monolithic ceramic electronic part, the substrate ceramic contains a major component expressed by the general formula Axe2x80x2Bxe2x80x2O3 in which Axe2x80x2 represents Ba or alternatively Ba partially substituted by at least one of Ca and Sr, and Bxe2x80x2 represents Ti or alternatively Ti partially substituted by at least one of Zr and Hf, and the inner conductor films are made of a sintered material produced by firing the above-described conductive paste.
Preferably, the monolithic ceramic electronic part is a monolithic ceramic capacitor. In this case, the inner conductor films are arranged via the ceramic layers in such a manner that a static capacitance can be generated, and the part further comprises outer electrodes formed on the outer surface of the sintered laminate in such a manner that the outer electrodes are electrically connected to specific ones of the inner conductor films to use the static capacitance.